The monitoring of volume blood flow through a vessel or other conduit of a patient is important for a number of reasons. During surgery, it is beneficial to monitor blood flow of the patient, among other vital signs that are monitored, to enhance the safety of the patient. Cardiovascular disorders can be better diagnosed and assessed through observation and study of volume blood flow. Restricted volume blood flow, including the degree thereof, is indicative of some form of cardiovascular disease. Flow measurements are also important when there has been a loss of blood and when the body has been subject or exposed to infection, metabolic disease, and unwanted drug/anesthetic effects.
Techniques have been previously devised for measuring volume blood flow. Traditional methods include invasive steps in which there is some disruption or alteration of the vascular system to be measured. Invasive approaches are unsuitable for a variety of reasons, including patient discomfort, greater risk because of the invasive steps that are required, and the need to use sterilized and expensive, non-reusable medical devices, such as catheters.
To overcome the drawbacks associated with invasive techniques, non-invasive methods have been advanced. These systems typically rely on the use of ultrasound and Doppler techniques. Basically, the velocity of the blood flow through a lumen is determined and that is multiplied by the cross-sectional or projected area of the lumen at the point of interest to determine volume blood flow.
A number of different approaches have been developed or proposed for determining volume blood flow. In U.S. Pat. No. 4,067,236 to Hottinger, issued Jan. 10, 1978, and entitled "Method and System for Unambiguous Measurement of Volume Flow," a Doppler system is disclosed in which flow velocity is determined as a function of the centroid or first moment of the Doppler power spectrum. In one embodiment disclosed in this patent, the lumen projected area is found using the ratios of returned Doppler power from two transducer elements. Circuitry is also described for providing information relating to the direction of blood flow. U.S. Pat. No. 4,431,936 to Fu et al., issued Feb. 14, 1984, and entitled "Transducer Structure for Generating Uniform and Focused Ultrasonic Beams and Applications Thereof" describes a transducer arrangement that, in one embodiment, includes outer and inner array elements for use in generating a wide beam and a narrow beam of ultrasonic energy. The Doppler information returning from the vessel of interest is used in determining volume blood flow. Another method for measuring flow volume that relies on a pair of transducer elements is found in U.S. Pat. No. 3,977,247 to Hassler, issued Aug. 31, 1976, and entitled "Arrangement for the Measurement of the Flow Volume of Flowing Media." The Hassler patent determines the lumen area of interest by using Doppler power associated with generated wide and narrow beams. The system disclosed in this patent also includes an indicator for displaying Doppler power signals from either the wide or narrow beam at any one time so that the highest intensity Doppler signal can be observed indicating that the narrower beam is passing through the blood vessel at the cross-sectional middle thereof. Further systems for measuring volume fluid flow using Doppler information are U.S. Pat. No. 4,062,237 to Fox, issued Dec. 13, 1977, and entitled "Cross Beam Ultrasonic Flowmeter," U.S. Pat. No. 3,554,030 to Peronneau, issued Jan. 12, 1971, and entitled "Recording Ultrasonic Flowmeter for Blood Vessels," and U.S. Pat. No. 3,498,290 to Shaw et al., issued Mar. 3, 1970, and entitled "Pulsed Doppler Volumetric Blood Flowmeter." These patents reveal various circuit elements and arrangements thereof for processing the received Doppler input including the use of multiplying circuits, band pass filters, amplitude limiters, and range gating circuitry. Another apparatus for measuring blood flow velocity is described in U.S. Pat. No. 4,593,700 to Hayakawi et al., issued June 10, 1986, and entitled "Ultrasonic Wave Blood Flow Meter." The disclosed apparatus includes orthogonal phase detection circuitry for providing Doppler information signals which have their phases shifted by 90.degree. from each other.
Despite the variety of systems that have been devised and which rely on non-invasive techniques, drawbacks to the use of such systems in a clinical environment still exist. In that regard, a volume blood flow measuring system that incorporates the features of providing highly accurate volume fluid flow-related measurements, while being relatively inexpensive and easy to use would be very beneficial to medical diagnosticians and others interested in obtaining information relating to volume fluid flow.